Hybrid nanocomposite materials are a combination of organic phase and inorganic phase molecules, wherein at least one phase is in the order of nanometers. Advantageously, hybrid materials exhibit synergistic properties, i.e., properties that are greater than the sum of the properties of each individual phase. In recent years, there has been considerable interest in the development of hybrid organic-inorganic nanocomposites that may improve the properties of paints and architectural coatings including dirt, water, and scratch resistance. Of particular interest has been the development of hybrids comprising latex polymers and nanosilica, given that silica, also known as silicon dioxide (SiO2), is well-known for its mechanical strength. See Stephen Krieger et al., “Colloidal Silica-Latex Polymer Nanocomposites for High Performance Coatings,” JCT Coatings Tech., 26-30 (August 2008).
Different processes for combining latex polymers with nanosilica are known in the paint and architectural coating art, but each known process suffers from a deficiency, e.g., lack of adequate bonding between the organic and inorganic phases, low solid content, and low silica content. The various known methods have not been able to achieve improvements, because the art of developing nanocomposites is unpredictable. As explained by Krieger et al., “There are various methods to make silica/organic nano composites, but in order to achieve the desired improvement by adding nanoparticles, the targeted organic polymer must display specific interaction tendencies with the inorganic particles.” Id. at p. 27.
One known process in the art is related in U.S. Pat. No. 6,455,219, which discloses a process for preparing a latex polymer, with colloidal silica, for use in a toner. In the process, monomers are pre-mixed with a nanosilica dispersion and the polymer is prepared through a semi-continuous process. The process yields polymer/nanosilica with a significant amount of coagulum, and forms a highly viscous and unstable product at high solid content. In addition, the emulsion polymer is not a nanocomposite because the inorganic silica particles are not covalently bonded to organic polymers.
U.S. Pat. No. 6,756,437 discloses a process of making a polymer-nanosilica dispersion by using a cationic surfactant. Specifically, the halogen-containing polymers such as polyvinyl chloride are mixed with silicates to yield polymers in which the organic and inorganic phases are not linked by covalent bonds. The process yields a product with a low solid content of about 10% wt. At high solid content, the process will form an unstable product and generate a large amount of coagulum.
U.S. Pat. No. 5,856,379 discloses a process of making a polymer comprising nanosilica particles. The nanosilica particles are treated with silane monomers at a pH around 5-6. The polymerization was carried out at the same pH conditions. The process yields unstable products, either in silica surface treatment or polymerization, that yield high content of coagulum or cause gellation of the batch.
Experiments conducted in accordance with the teachings of the prior art references discussed in the above three paragraphs show that coagulum levels are high.
U.S. Pat. Pub. No. 2004/0151910 discloses an organic-inorganic composite containing an inorganic particle having a plurality of polymer particles attached to the inorganic particle and a polymer layer encapsulating the attached polymer particles. However, the composite particles are not nano-sized, and are not formed from silane monomers.
U.S. Pat. Pub. No. 2006/0134420 relates to nanometric or mesoscopic dissymmetric particles, and to a method for preparing the same. More particularly, the particles have an inorganic part A and a spherical organic part B bound by physicochemical or covalent interactions. However, the inorganic particle content is low. The data for Example 1 of this reference indicate the initial silica concentration ranges from 0.25% to 16%.
Thus, there remains a need for a hybrid organic-inorganic nanocomposite molecules comprising discrete inorganic silica nanoparticles that are covalently bonded to organic polymers, and further comprising high solid content and high silica content with low coagulum.